2,496 research outputs found
Aggregation-based Multilevel Methods for Lattice QCD
In Lattice QCD computations a substantial amount of work is spent in solving
the Dirac equation. In the recent past it has been observed that conventional
Krylov solvers tend to critically slow down for large lattices and small quark
masses. We present a Schwarz alternating procedure (SAP) multilevel method as a
solver for the Clover improved Wilson discretization of the Dirac equation.
This approach combines two components (SAP and algebraic multigrid) that have
separately been used in lattice QCD before. In combination with a bootstrap
setup procedure we show that considerable speed-up over conventional Krylov
subspace methods for realistic configurations can be achieved.Comment: Talk presented at the XXIX International Symposium on Lattice Field
Theory, July 10-16, 2011, Lake Tahoe, Californi
Fluctuations and correlations in high temperature QCD
We calculate second- and fourth-order cumulants of conserved charges in a
temperature range stretching from the QCD transition region towards the realm
of (resummed) perturbation theory. We perform lattice simulations with
staggered quarks; the continuum extrapolation is based on in
the crossover-region and at higher temperatures. We find that
the Hadron Resonance Gas model predictions describe the lattice data rather
well in the confined phase. At high temperatures (above 250 MeV) we find
agreement with the three-loop Hard Thermal Loop results.Comment: 18 pages revtex, 13 figure
Disconnected contributions to the spin of the nucleon
The spin decomposition of the proton is a long-standing topic of much
interest in hadronic physics. Lattice QCD has had much success in calculating
the connected contributions to the quark spin. However, complete calculations,
which necessarily involve gluonic and strange-quark contributions, still
present some challenges. These "disconnected" contributions typically involve
small signals hidden against large statistical backgrounds and rely on
computationally intensive stochastic techniques. In this work we demonstrate
how a Feynman-Hellmann approach may be used to calculate such quantities, by
measuring shifts in the proton energy arising from artificial modifications to
the QCD action. We find a statistically significant non-zero result for the
disconnected quark spin contribution to the proton of about -5% at a pion mass
of 470 MeV
A review of High Performance Computing foundations for scientists
The increase of existing computational capabilities has made simulation
emerge as a third discipline of Science, lying midway between experimental and
purely theoretical branches [1, 2]. Simulation enables the evaluation of
quantities which otherwise would not be accessible, helps to improve
experiments and provides new insights on systems which are analysed [3-6].
Knowing the fundamentals of computation can be very useful for scientists, for
it can help them to improve the performance of their theoretical models and
simulations. This review includes some technical essentials that can be useful
to this end, and it is devised as a complement for researchers whose education
is focused on scientific issues and not on technological respects. In this
document we attempt to discuss the fundamentals of High Performance Computing
(HPC) [7] in a way which is easy to understand without much previous
background. We sketch the way standard computers and supercomputers work, as
well as discuss distributed computing and discuss essential aspects to take
into account when running scientific calculations in computers.Comment: 33 page
A note on entropic uncertainty relations of position and momentum
We consider two entropic uncertainty relations of position and momentum
recently discussed in literature. By a suitable rescaling of one of them, we
obtain a smooth interpolation of both for high-resolution and low-resolution
measurements respectively. Because our interpolation has never been mentioned
in literature before, we propose it as a candidate for an improved entropic
uncertainty relation of position and momentum. Up to now, the author has
neither been able to falsify nor prove the new inequality. In our opinion it is
a challenge to do either one.Comment: 2 pages, 2 figures, 2 references adde
Quantitative Description of Pedestrian Dynamics with a Force based Model
This paper introduces a space-continuous force-based model for simulating
pedestrian dynamics. The main interest of this work is the quantitative
description of pedestrian movement through a bottleneck. Measurements of flow
and density will be presented and compared with empirical data. The results of
the proposed model show a good agreement with empirical data. Furthermore, we
emphasize the importance of volume exclusion in force-based models.Comment: 4 pages, 7 figures, 2009 IEEE/WIC/ACM International Joint Conferences
on Web Intelligence and Intelligent Agent Technologies (WI-IAT 2009), 15-18
September 2009, in Milano, Italy, 200
Charm quark effects on the strong coupling extracted from the static force
We compute the fermionic contribution to the strong coupling
extracted from the static force in Lattice QCD up to order in
perturbation theory. This allows us to subtract the leading fermionic lattice
artifacts from recent determinations of produced in simulations
of two dynamical charm quarks. Moreover, by using a suitable parametrization of
the -function, we can evaluate the charm loop effects on
in the continuum limit.Comment: 8 pages, 2 figures; Proceedings of the 35th International Symposium
on Lattice Field Theory, Granada, Spai
Online Fault Classification in HPC Systems through Machine Learning
As High-Performance Computing (HPC) systems strive towards the exascale goal,
studies suggest that they will experience excessive failure rates. For this
reason, detecting and classifying faults in HPC systems as they occur and
initiating corrective actions before they can transform into failures will be
essential for continued operation. In this paper, we propose a fault
classification method for HPC systems based on machine learning that has been
designed specifically to operate with live streamed data. We cast the problem
and its solution within realistic operating constraints of online use. Our
results show that almost perfect classification accuracy can be reached for
different fault types with low computational overhead and minimal delay. We
have based our study on a local dataset, which we make publicly available, that
was acquired by injecting faults to an in-house experimental HPC system.Comment: Accepted for publication at the Euro-Par 2019 conferenc
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